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Old Jun 17th 2015, 02:19 PM   #1
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Why do photons change colour?

If we shine white light on to a blue surface then only blue light is reflected and so we see the surface as blue. However, I am wondering about the bigger picture.

I understand white is made up of multiple light frequencies, red, blue and green. These have different energies and we know E = h*f where f is the frequency of the electromagnetic wave and h is a constant.

Now if lights of different frequencies are directed to a blue surface then the only way, that I know of, whereby these photons will be reflected is if the interact with some of those subatomic particles that make up the blue surface. The way I imagine it happening is that a photon interacts with an electron and in many cases is absorbed by the electron and the electron jumps to another energy level in its orbital. Which energy level it jumps to must be exactly equal to the energy of the incoming photon. Later on the electron may jump back to its original energy level and emit a photon and I presume this what makes up the reflected light.

The question I have about this whole process is why are all the photons reflected "blue" rather than a mixture of frequencies?
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Old Jun 17th 2015, 02:41 PM   #2
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In normal reflection, the colour does not change.
If you shine Red light onto a Blue surface, it will look Black
because all the red photons will be absorbed.
Similarly for blue photons and a red surface, etc.

You can see this effect clearly under sodium street lights,
Blue cars will look the same as Black cars.

Note that the blue surface looks blue because it does NOT absorb blue light, but it does absorb the other colours

However, there is an effect where the photons are absorbed, and then re-radiated at a different frequency, this is florescence.
Higher energy photons are absorbed by the electrons in the surface and they "jump up" 2 (or more) energy levels in one step.
They then jump back down the energy levels in separate individual steps, at each step releasing a photon of lower energy (different colour) than the single original energising photon.
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Last edited by MBW; Jun 17th 2015 at 02:54 PM.
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Old Jun 17th 2015, 02:50 PM   #3
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Just reviewed my last post, and it raised this question in my head...

Were does the energy of the absorbed red photons go in a blue surface
(or blue photons in a read surface etc.)
I guess that the energy must heat up the surface, which then re-radiates as infra-red.
Is this a form of florescence, just at a lower (non-visible) frequency?
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Old Jun 17th 2015, 04:26 PM   #4
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MBW - yes, if a surface absorbs one color (frequency) and not others then the energy of that color is absorbed, which mat result in heat or other uses of that energy. Example: in photosynthesis the plant absorbs energy from the sun in the red and violet ends of the spectrum, using that energy to create sugars. The green part of the spectrum is not used, and so is emitted back to your eye. That's why most plant leaves look green - because green light is no good for photosynthesis.
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Old Jun 18th 2015, 10:40 PM   #5
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Interesting point about plant leaves. How much heat do plants radiate? If its fairly low there must be significant addition to plant potential energy as a result of this process.
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Old Jun 19th 2015, 05:47 AM   #6
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Not trying to be facetious. but what do you mean by "heat?" Living plants are indeed net consumers of solar energy (otherwise they wouldn't grow) - I assume that's what you mean by "addition to plant potential energy." As with all processes the conversion of CO2 and H20 into sugar through photosynthesis is not 100% efficient, so it would seem to me that some heat must be generated as a result. But the amount of that heat, plus the energy of light not used in photosynthesis and thus re-radiated, is less than the amount of solar energy that falls on the plant.

Of course after the plant dies and starts to decay the breaking-down of its tissues generates more heat. If you stick your hand deep into a compost pile you'll find it's warm to the touch.
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